Many sources of power such as batteries are of such nature that for diverse reasons the voltage supplied to the load varies with use. This may seriously affect the performance of the device acting as the load and the overall power consumption.
Voltage regulation is generally accomplished in such circuits by putting a suitable and desirable impedance in the line. This provides a drop in voltage which increases with the current and hence reduces the voltage applied to the load. This voltage drop may be increased by a circuit paralleling the load which decreases in resistance as the voltage increases, thus increasing the overall current and the voltage drop in the series resistance. This maintains a reduced voltage.
The disadvantage of this method resides in the fact that there is a substantial loss in wattage while the voltage is high, but losses in the series resistance continue when the voltage of the source drops.